Hmmm ... I joined the Navy in October, '73, to get the GI Bill. Plan was to stay my four years and get out; ended up doing 9 1/2, and got out reluctantly because of my kids 9 years, 5 months, and 22 days later.
What good did it do me? I can't begin to estimate it. Taught me discipline, taught me structure, taught me how to work the system. Let me see a bit of the world, let me meet people from all walks of life and every area of the country and just about every ethnic mix you could imagine.
Bad? Can't think of anything. I had a negative attitude toward authority before I went in, and still do
All services have their advantages, so the following is a pretty broad generalization. If a young person is looking for physical challenges, the USMC is the way to go, followed by the Army. For mental discipline the Navy is probably the best, but I might be biased. For technically-oriented training, however, the USAF has the stuff that is most likely to carry into civilian life.
BTW, if he has enought ROTC to get credit upon enlistment, it should carry over into the other services. He should talk to ALL the recruiters before he makes a decision, and he should think about it for a couple of weeks before he makes a final decision. Tell him not to be afraid to ask questions.